configure: fix ncurses lib symbol checking

When probing symbols provided by ncurses, use the sublibs that pkg-config
reported in case they're needed (like when libtinfo is a sep library).

URL: https://savannah.gnu.org/bugs/?49614

configure.ac

@@ -551,8 +551,12 @@ else
     AC_MSG_RESULT([Using $CURSES_LIB_NAME as the curses library])
 fi
 
-AC_CHECK_LIB([$CURSES_LIB_NAME], use_default_colors, AC_DEFINE(HAVE_USE_DEFAULT_COLORS, 1, [Define this if your curses library has the use_default_colors() command.]))
-AC_CHECK_LIB([$CURSES_LIB_NAME], key_defined, AC_DEFINE(HAVE_KEY_DEFINED, 1, [Define this if your curses library has the key_defined() command.]))
+AC_CHECK_LIB([$CURSES_LIB_NAME], [use_default_colors],
+	     [AC_DEFINE(HAVE_USE_DEFAULT_COLORS, 1, [Define this if your curses library has the use_default_colors() command.])],
+	     [], [$CURSES_LIB])
+AC_CHECK_LIB([$CURSES_LIB_NAME], [key_defined],
+	     [AC_DEFINE(HAVE_KEY_DEFINED, 1, [Define this if your curses library has the key_defined() command.])],
+	     [], [$CURSES_LIB])
 
 dnl Parse any configure options.